Detection systems for power equipment

ABSTRACT

Improved methods to detect when a human body contacts a predetermined portion of a machine are disclosed. The methods distinguish contact with a person from contact with other materials. The methods are particularly applicable in woodworking equipment such as table saws to distinguish contact between a person and the blade of the saw from contact between the blade and wet or green wood.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/027,600, filed Dec. 31, 2004, issuing as U.S. Pat. No. 7,536,238 onMay 19, 2009, which in turn claims the benefit of and priority from U.S.Provisional Patent Application Ser. No. 60/533,791, filed Dec. 31, 2003,the disclosures of which are herein incorporated by reference.

COMPUTER PROGRAM LISTING APPENDIX

Two compact discs, each containing a computer program listing that isone implementation of the methods and systems described herein, arebeing submitted herewith as a Computer Program Listing Appendix. Thecompact discs are identified as “Copy 1” and “Copy 2” and they areidentical. The program listing is stored on each compact disc as oneASCII text file entitled “program.asm”. The date of creation of thefiles is Dec. 31, 2003, and the size of each file is 292 kbytes. Thematerial on the compact discs is hereby incorporated by reference.

COPYRIGHT NOTICE

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent file or records, but otherwise reserves all copyrightrights whatsoever.

FIELD

The present invention relates to safety systems for power tools such astable saws, miter saws, band saws, hand-held circular saws, jointers,shapers, routers, up-cut saws, and other machinery. More particularly,the present invention relates to detecting contact between a human and aportion of a piece of power equipment.

BACKGROUND

Safety systems may be employed with power equipment to minimize the riskof injury when using the equipment. Some safety systems include anelectronic system to detect the occurrence of a dangerous condition anda reaction system to minimize any possible injury from the dangerouscondition. For instance, some safety systems attempt to detect when ahuman body contacts or comes into dangerous proximity to a predeterminedportion of a machine, such as detecting when a user's hand touches themoving blade of a saw. When that dangerous condition is detected, thesafety system reacts to minimize injury.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a machine with a fast-actingsafety system.

FIG. 2 is a schematic diagram of an exemplary safety system in thecontext of a machine having a circular blade.

FIG. 3 is a flowchart of a method of detecting contact.

FIG. 4 is a flowchart of another method of detecting contact.

FIG. 5 is a flowchart of still another method of detecting contact.

FIG. 6 is a diagram of connections between various components in amachine.

FIG. 7 is a diagram of a voltage regulation circuit that may be usedwith a cartridge as described herein.

FIG. 8 is a diagram of a signal digital processing microcontroller usedto control the circuits described herein.

FIG. 9 is a diagram of a circuit that can be used to program the digitalsignal processing microcontroller shown in FIG. 8 from an externalsource.

FIG. 10 is a diagram of a circuit that provides a cycle time for thedigital signal processing microcontroller shown in FIG. 8.

FIG. 11 is a diagram of a circuit used to hold the digital signalprocessing microcontroller shown in FIG. 8 in reset unless a powersupply is at proper operating voltage.

FIG. 12 is a diagram of a phase lock loop used with the digital signalprocessing microcontroller shown in FIG. 8.

FIG. 13 is a diagram of a switch that can be used with the digitalsignal processing microcontroller shown in FIG. 8.

FIG. 14 is a diagram of a circuit for a 500 kHz driver.

FIG. 15 is a diagram of an integrator circuit.

FIG. 16 is a diagram of a circuit that can be used to detectelectrically the spacing between a blade and a brake pawl.

FIG. 17 is a diagram of a circuit used to power motor control relays.

FIG. 18 is a diagram of a circuit used in a firing subsystem asdescribed herein.

FIG. 19 is a diagram of a power supply circuit.

FIG. 20 is a diagram showing connections between components in machinesusing smaller motors.

FIG. 21 is a diagram showing connections between components in machinesusing larger motors.

FIG. 22 is a diagram showing an alternative digital signal processorimplementation.

DETAILED DESCRIPTION

A machine that incorporates a safety system to detect and react to humancontact with the machine is shown schematically in FIG. 1 and indicatedgenerally at 10. Machine 10 may be any of a variety of differentmachines, such as table saws, miter saws, band saws, jointers, shapers,routers, hand-held circular saws, up-cut saws, sanders, etc. Machine 10includes an operative structure 12 having a working or cutting tool 14and a motor assembly 16 adapted to drive the cutting tool. Machine 10also includes a safety system 18 configured to minimize the potential ofa serious injury to a person using the machine. Safety system 18 isadapted to detect the occurrence of one or more dangerous conditionsduring use of the machine. If such a dangerous condition is detected,safety system 18 is adapted to engage operative structure 12 to limitany injury to the user caused by the dangerous condition.

Machine 10 also includes a suitable power source 20 to provide power tooperative structure 12 and safety system 18. Power source 20 may be anexternal power source such as line current, or an internal power sourcesuch as a battery. Alternatively, power source 20 may include acombination of both external and internal power sources. Furthermore,power source 20 may include two or more separate power sources, eachadapted to power different portions of machine 10.

It will be appreciated that operative structure 12 may take any one ofmany different forms. For example, operative structure 12 may include astationary housing configured to support motor assembly 16 in drivingengagement with cutting tool 14. Alternatively, operative structure 12may include one or more transport mechanisms adapted to convey a workpiece toward and/or away from cutting tool 14.

Motor assembly 16 includes at least one motor adapted to drive cuttingtool 14. The motor may be either directly or indirectly coupled to thecutting tool, and may also be adapted to drive work piece transportmechanisms. The particular form of cutting tool 14 will vary dependingupon the various embodiments of machine 10. For example, cutting tool 14may be a single, circular rotating blade having a plurality of teethdisposed along the perimetrical edge of the blade. Alternatively, thecutting tool may be a plurality of circular blades, such as a dado bladeor dado stack, or some other type of blade or working tool.

Safety system 18 includes a detection subsystem 22, a reaction or dangermitigation subsystem 24 and a control subsystem 26. Control subsystem 26may be adapted to receive inputs from a variety of sources includingdetection subsystem 22, reaction subsystem 24, operative structure 12and motor assembly 16. The control subsystem may also include one ormore sensors adapted to monitor selected parameters of machine 10. Inaddition, control subsystem 26 typically includes one or moreinstruments operable by a user to control the machine. The controlsubsystem is configured to control machine 10 in response to the inputsit receives.

Detection subsystem 22 is configured to detect one or more dangerous ortriggering conditions during use of machine 10. For example, thedetection subsystem may be configured to detect that a portion of theuser's body is dangerously close to or in contact with a portion ofcutting tool 14. As another example, the detection subsystem may beconfigured to detect the rapid movement of a workpiece due to kickbackby the cutting tool, as is described in U.S. patent application Ser. No.09/676,190, the disclosure of which is herein incorporated by reference.In some embodiments, detection subsystem 22 may inform control subsystem26 of the dangerous condition, which then activates reaction subsystem24. In other embodiments, the detection subsystem may be adapted toactivate the reaction subsystem directly.

Once activated in response to a dangerous condition, reaction subsystem24 is configured to engage operative structure 12 quickly to preventserious injury to the user. It will be appreciated that the particularaction to be taken by reaction subsystem 24 will vary depending on thetype of machine 10 and/or the dangerous condition that is detected. Forexample, reaction subsystem 24 may be configured to do one or more ofthe following: stop the movement of cutting tool 14, disconnect motorassembly 16 from power source 20, place a barrier between the cuttingtool and the user, or retract the cutting tool from its operatingposition, etc. The reaction subsystem may be configured to take acombination of steps to protect the user from serious injury. Placementof a barrier between the cutting tool and teeth is described in moredetail in U.S. Patent Application Publication No. 2002/0017183 A1,entitled “Cutting Tool Safety System,” the disclosure of which is hereinincorporated by reference. Retracting the cutting tool is described inmore detail in U.S. Patent Application Publication No. 2002/0017181 A1,entitled “Retraction System for Use in Power Equipment,” and U.S. PatentApplication Ser. No. 60/452,159, filed Mar. 5, 2003, entitled“Retraction System and Motor Position for Use With Safety Systems forPower Equipment,” the disclosures of which are herein incorporated byreference.

The configuration of reaction subsystem 24 typically will vary dependingon which action or actions are taken. In the exemplary embodimentdepicted in FIG. 1, reaction subsystem 24 is configured to stop themovement of cutting tool 14 and includes a brake mechanism 28, a biasingmechanism 30, a restraining mechanism 32, and a release mechanism 34.Brake mechanism 28 is adapted to engage operative structure 12 under theurging of biasing mechanism 30. During normal operation of machine 10,restraining mechanism 32 holds the brake mechanism out of engagementwith the operative structure. However, upon receipt of an activationsignal by reaction subsystem 24, the brake mechanism is released fromthe restraining mechanism by release mechanism 34, whereupon, the brakemechanism quickly engages at least a portion of the operative structureto bring the cutting tool to a stop.

It will be appreciated by those of skill in the art that the exemplaryembodiment depicted in FIG. 1 and described above may be implemented ina variety of ways depending on the type and configuration of operativestructure 12. Turning attention to FIG. 2, one example of the manypossible implementations of safety system 18 is shown. System 18 isconfigured to engage an operative structure having a circular blade 40mounted on a rotating shaft or arbor 42. Blade 40 includes a pluralityof cutting teeth (not shown) disposed around the outer edge of theblade. As described in more detail below, braking mechanism 28 isadapted to engage the teeth of blade 40 and stop the rotation of theblade. U.S. Patent Application Publication No. 2002/0017175 A1, entitled“Translation Stop For Use In Power Equipment,” the disclosure of whichis herein incorporated by reference, describes other systems forstopping the movement of the cutting tool. U.S. Patent ApplicationPublication No. 2002/0017184 A1, entitled “Table Saw With ImprovedSafety System,” U.S. Patent Application Publication No. 2002/0017179 A1,entitled “Miter Saw With Improved Safety System,” U.S. PatentApplication Publication No. 2002/0059855 A1, entitled “Miter Saw withImproved Safety System,” U.S. Patent Application Publication No.2002/0056350 A1, entitled “Table Saw With Improved Safety System,” U.S.Patent Application Publication No. 2002/0059854 A1, entitled “Miter SawWith Improved Safety System,” U.S. Patent Application Publication No.2002/0056349 A1, entitled “Miter Saw With Improved Safety System,” U.S.Patent Application Publication No. 2002/0056348 A1, entitled “Miter SawWith Improved Safety System,” and U.S. Patent Application PublicationNo. 2002/0066346 A1, entitled “Miter Saw With Improved Safety System,”U.S. Patent Application Publication No. 2003/0015253 A1, entitled“Router With Improved Safety System,” U.S. Patent ApplicationPublication No. 2002/0170400 A1, entitled “Band Saw With Improved SafetySystem,” U.S. Patent Application Publication No. 2003/0019341 A1,entitled “Safety Systems for Band Saws,” U.S. Patent ApplicationPublication No. 2003/0056853 A1, entitled “Router With Improved SafetySystem,” U.S. Provisional Patent Application Ser. No. 60/406,138,entitled “Miter Saw With Improved Safety System,” U.S. ProvisionalPatent Application Ser. No. 60/496,550, entitled “Table Saws With SafetySystems,” and U.S. Provisional Patent Application Ser. No. 60/533,811,entitled “Improved Table Saws with Safety Systems,” the disclosures ofwhich are all herein incorporated by reference, describe safety system18 in the context of particular types of machines.

In the exemplary implementation, detection subsystem 22 is adapted todetect the dangerous condition of the user coming into contact withblade 40. The detection subsystem includes a sensor assembly, such ascontact detection plates 44 and 46, capacitively coupled to blade 40 todetect any contact between the user's body and the blade. Typically, theblade, or some larger portion of cutting tool 14 is electricallyisolated from the remainder of machine 10. Alternatively, detectionsubsystem 22 may include a different sensor assembly configured todetect contact in other ways, such as optically, resistively, etc. Inany event, the detection subsystem is adapted to transmit a signal tocontrol subsystem 26 when contact between the user and the blade isdetected. Various exemplary embodiments and implementations of detectionsubsystem 22 are described in more detail in U.S. Patent ApplicationPublication No. 2002/0017176 A1, entitled “Detection System For PowerEquipment,” U.S. Patent Application Publication No. 2002/0017336 A1,entitled “Apparatus And Method For Detecting Dangerous Conditions InPower Equipment,” U.S. Patent Application Publication No. 2002/0069734A1, entitled “Contact Detection System for Power Equipment,” U.S. PatentApplication Publication No. 2002/0190581 A1, entitled “Apparatus andMethod for Detecting Dangerous Conditions in Power Equipment,” U.S.Patent Application Publication No. 2003/0002942 A1, entitled “DiscreteProximity Detection System,” and U.S. Patent Application Publication No.2003/0090224 A1, entitled “Detection System for Power Equipment,” thedisclosures of which are herein incorporated by reference.

Control subsystem 26 includes one or more instruments 48 that areoperable by a user to control the motion of blade 40. Instruments 48 mayinclude start/stop switches, speed controls, direction controls,light-emitting diodes, etc. Control subsystem 26 also includes a logiccontroller 50 connected to receive the user's inputs via instruments 48.Logic controller 50 is also connected to receive a contact detectionsignal from detection subsystem 22. Further, the logic controller may beconfigured to receive inputs from other sources (not shown) such asblade motion sensors, work piece sensors, etc. In any event, the logiccontroller is configured to control operative structure 12 in responseto the user's inputs through instruments 48. However, upon receipt of acontact detection signal from detection subsystem 22, the logiccontroller overrides the control inputs from the user and activatesreaction subsystem 24 to stop the motion of the blade. Various exemplaryembodiments and implementations of control subsystem 26, and componentsthat may be used in control system 26, are described in more detail inU.S. Patent Application Publication No. 2002/0020262 A1, entitled “LogicControl For Fast Acting Safety System,” U.S. Patent ApplicationPublication No. 2002/0017178 A1, entitled “Motion Detecting System ForUse In Safety System For Power Equipment,” U.S. Patent ApplicationPublication No. 2003/0058121 A1, entitled “Logic Control With Test Modefor Fast-Acting Safety System,” U.S. Provisional Patent Application Ser.No. 60/496,568, entitled “Motion Detecting System for use in a SafetySystem for Power Equipment,” and U.S. Provisional Patent ApplicationSer. No. 60/533,598, entitled “Switch Box for Power Tools with SafetySystems,” the disclosures of which are herein incorporated by reference.

In the exemplary implementation, brake mechanism 28 includes a pawl 60mounted adjacent the edge of blade 40 and selectively moveable to engageand grip the teeth of the blade. Pawl 60 may be constructed of anysuitable material adapted to engage and stop the blade. As one example,the pawl may be constructed of a relatively high strength thermoplasticmaterial such as polycarbonate, ultrahigh molecular weight polyethylene(UHMW) or Acrylonitrile Butadiene Styrene (ABS), etc., or a metal suchas fully annealed aluminum, etc. It will be appreciated that theconstruction of pawl 60 may vary depending on the configuration of blade40. In any event, the pawl is urged into the blade by a biasingmechanism in the form of a spring 66. In the illustrative embodimentshown in FIG. 2, pawl 60 is pivoted into the teeth of blade 40. Itshould be understood that sliding or rotary movement of pawl 60 mightalso be used. The spring is adapted to urge pawl 60 into the teeth ofthe blade with sufficient force to grip the blade and quickly bring itto a stop.

The pawl is held away from the edge of the blade by a restrainingmechanism in the form of a fusible member 70. The fusible member isconstructed of a suitable material adapted to restrain the pawl againstthe bias of spring 66, and also adapted to melt under a determinedelectrical current density. Examples of suitable materials for fusiblemember 70 include NiChrome wire, stainless steel wire, etc. The fusiblemember is connected between the pawl and a contact mount 72. Preferably,fusible member 70 holds the pawl relatively close to the edge of theblade to reduce the distance the pawl must travel to engage the blade.Positioning the pawl relatively close to the edge of the blade reducesthe time required for the pawl to engage and stop the blade. Typically,the pawl is held approximately 1/32-inch to ¼-inch from the edge of theblade by fusible member 70; however other pawl-to-blade spacings mayalso be used.

Pawl 60 is released from its unactuated, or cocked, position to engageblade 40 by a release mechanism in the form of a firing subsystem 76.The firing subsystem is coupled to contact mount 72, and is configuredto melt fusible member 70 by passing a surge of electrical currentthrough the fusible member. Firing subsystem 76 is coupled to logiccontroller 50 and activated by a signal from the logic controller. Whenthe logic controller receives a contact detection signal from detectionsubsystem 22, the logic controller sends an activation signal to firingsubsystem 76, which melts fusible member 70, thereby releasing the pawlto stop the blade. Various exemplary embodiments and implementations ofreaction subsystem 24 are described in more detail in U.S. PatentApplication Publication No. 2002/0020263 A1, entitled “Firing SubsystemFor Use In A Fast-Acting Safety System,” U.S. Patent ApplicationPublication No. 2002/0020271 A1, entitled “Spring-Biased Brake Mechanismfor Power Equipment,” U.S. Patent Application Publication No.2002/0017180 A1, entitled “Brake Mechanism For Power Equipment,” U.S.Patent Application Publication No. 2002/0059853 A1, entitled “Power SawWith Improved Safety System,” U.S. Patent Application Publication No.2002/0020265 A1, entitled “Translation Stop For Use In Power Equipment,”U.S. Patent Application Publication No. 2003/0005588 A1, entitled“Actuators For Use in Fast-Acting Safety Systems,” and U.S. PatentApplication Publication No. 2003/0020336 A1, entitled “Actuators For UseIn Fast-Acting Safety Systems,” the disclosures of which are all hereinincorporated by reference.

It will be appreciated that activation of the brake mechanism willrequire the replacement of one or more portions of safety system 18. Forexample, pawl 60 and fusible member 70 typically must be replaced beforethe safety system is ready to be used again. Thus, it may be desirableto construct one or more portions of safety system 18 in a cartridgethat can be easily replaced. For example, in the exemplaryimplementation depicted in FIG. 2, safety system 18 includes areplaceable cartridge 80 having a housing 82. Pawl 60, spring 66,fusible member 70 and contact mount 72 are all mounted within housing82. Alternatively, other portions of safety system 18 may be mountedwithin the housing. In any event, after the reaction system has beenactivated, the safety system can be reset by replacing cartridge 80. Theportions of safety system 18 not mounted within the cartridge may bereplaced separately or reused as appropriate. Various exemplaryembodiments and implementations of a safety system using a replaceablecartridge, and various brake pawls, are described in more detail in U.S.Patent Application Publication No. 2002/0020261 A1, entitled“Replaceable Brake Mechanism For Power Equipment,” U.S. PatentApplication Publication No. 2002/0017182 A1, entitled “Brake PositioningSystem,” U.S. Patent Application Publication No. 2003/0140749 A1,entitled “Brake Pawls for Power Equipment,” U.S. Provisional PatentApplication Ser. No. 60/496,574, entitled “Brake Cartridges for PowerEquipment,” filed Aug. 20, 2003, and U.S. Provisional Patent ApplicationSer. No. 60/533,575, entitled “Brake Cartridges and Mounting Systems forBrake Cartridges,” the disclosures of which are all herein incorporatedby reference.

While one particular implementation of safety system 18 has beendescribed, it will be appreciated that many variations and modificationsare possible. Many such variations and modifications are described inU.S. Patent Application Publication No. 2002/0170399 A1, entitled“Safety Systems for Power Equipment,” U.S. Patent ApplicationPublication No. 2003/0037651, entitled “Safety Systems for PowerEquipment,” and U.S. Patent Application Publication No. 2003/0131703 A1,entitled “Apparatus and Method for Detecting Dangerous Conditions inPower Equipment,” the disclosures of which are herein incorporated byreference.

In some applications, detection subsystem 22 detects contact between ahuman and a predetermined portion of the machine and it is desirable todistinguish that contact from contact with other materials. For example,in a saw the detection subsystem may detect contact between a person andthe blade and distinguish that contact from contact between the bladeand other materials such as wet or green wood. It should also beunderstood that in many aspects of the inventions disclosed herein thedangerous condition detected could be dangerous proximity rather thanactual contact, even though much of the disclosure is particularly todetecting actual contact.

One method of distinguishing human contact from contact with othermaterials is shown generally in FIG. 3. First, as explained above, theelectrical impedance of the blade is monitored, such as by inducing anelectrical signal on the blade, which is represented by box 100 in FIG.3. The electrical impedance of the blade can be monitored in a varietyof ways, such as applying a fixed voltage to a drive electrode coupledto the blade and monitoring the voltage thereby induced on the blade.That voltage will drop as the apparent impedance of the blade drops.Alternatively, the amount of electrical current delivered to the bladefor a fixed drive voltage can be monitored and this current would go upas the impedance of the blade dropped. However the impedance of theblade is measured or monitored, the detection system looks for changesat 102. Such changes in electrical impedance will occur when the bladecuts into or contacts a person. However, the electrical impedance mayalso change due to other circumstances, such as when the blade contactsother materials, especially wet or green wood, or when the signal isexposed to electrical noise. It is desirable to distinguish the changein the electrical impedance caused by human contact from changes in theimpedance caused by other events; otherwise, the detection system mayfalsely detect contact with a human when it should not.

As one exemplary way to distinguish changes in the electrical impedance,the method converts the detected signal on the blade into valuesproportional to the peak-to-peak amplitude or RMS amplitude of thesignal, as shown at 104. In one implementation, the method discretelyconverts the detected signal into amplitude values every 6 microseconds.Of course, other time periods could be used, such as every 2 to 10microseconds or the conversion could even be done continuously. One wayof converting the detected signal is to integrate the absolute value ofthe detected signal about its average value, resulting in a data pointthat is proportional to the peak-to-peak amplitude of the detectedsignal.

Integrating the signal has the benefit of averaging out the effect ofnoise on the signal, resulting in a data point that is relativelyresistant to noise. For instance, it can be seen that if the signal usedfor contact detection has a frequency of 500 kHz, then three cycles ofthe signal will be integrated to generate the amplitude level. However,if there is a 10 MHz noise present, that noise will have relativelylittle effect on the measured amplitude value since it will besubstantially averaged out over the course of the integration.Similarly, if there is a low frequency noise on the signal that shiftsthe apparent DC level of the 500 khz somewhat, the absolute valueintegration will minimize the effect since one lobe of the sine wavewill be increased by the DC shift and the following lobe will belowered, with the overall effect of the DC offset on the measurement ofamplitude being substantially reduced, although not entirely eliminated.In other words, integrating the signal can be seen as a noise reductiontechnique. It will of course be understood that numerous othertechniques could be used to generate a value proportional to theamplitude of the signal, such as looking for the difference between themaximum and minimum values over some interval or even just looking forthe peak value over some time interval.

It should be noted that although the system for detecting contact isdescribed herein principally in terms of monitoring the amplitude of asignal on the blade, more generally, the system is monitoring theelectrical impedance of the blade and the amplitude of the signal isjust one measure of that impedance. The system could equally well beimplemented with values proportional to other characteristics of theimpedance, such as the phase of the signal on the blade, currentdelivered to the blade or any other variable related to the electricalimpedance of the blade. In reading the description herein, it should beunderstood that the values tracked could be based on any variablerelated to the electrical impedance of the blade.

At the time of integration, the signal on the blade may have increasedor decreased from an immediately prior integration, causing the value ofthe current data point to be larger or smaller than the value of theprior data point. The method compares the value of the currentintegrated data point to the last previously integrated data point tosee how the signal has changed, as shown at 106. This change issometimes referred to as dV/dt or the change in value of the data pointover time. This dV/dt value is typically referred to as a derivative orrate of change and is proportional to the rate of change of theamplitude of the signal on the blade. Again, this dV/dt value could becalculated or sampled at varying intervals from continuous to every 10microseconds or longer depending on how fast relevant changes in thesignal properties may occur.

In order to generate a value proportional to the cumulative orpersistent rate of change of the signal over some time interval, themethod takes the absolute value of dV/dt at 108, and then sequentiallyadds together a given number of such absolute values at 110. Forexample, the method may sum the prior 16 data points, which means thatif the method obtains a new data point every 6 microseconds, then themethod will look at a window of 96 microseconds. If the sum of the dV/dtvalues during that window is greater than a predetermined thresholdvalue, then the detection system concludes that the blade has contacteda person and the detection system will trigger the reaction system. Thepredetermined threshold value is set empirically based onexperimentation and observed test results and is dependent on theelectrical impedance of the blade and the various materials to bedistinguished, as well as the characteristics of the way the signal isinduced on and detected from the blade. The sum of the absolute valuesof dV/dt of these 16 data points may be referred to as a short sum. Itshould be understood that it is not necessary to take the absolute valueof the signal, but rather just the sum of the dV/dt values could becomputed, although it is believed that summing the absolute value of thedV/dt values provides better discrimination of human contact events fromother effects that can change the signal on the blade. Whether theabsolute value or unmodified values are summed, this scheme can be seenas looking at the accumulated time properties of a data stream relatedto the electrical impedance of the blade, in distinction to just lookingat an instantaneous value, such as the instantaneous rate of change.

The method preferably repeats continuously while the machine isfunctioning. Typically the method will repeat for each new data point.Thus, if a new data point is integrated every 6 microseconds then themethod would repeat every 6 microseconds. As the method repeats, thenewest absolute value of dV/dt replaces the oldest such value in the sumso that the window of time the short sum covers is continuously movingor sliding forward. In terms of processing, this can be accomplished bystoring the short sum in memory and adding to that sum each new datapoint while subtracting from that sum the oldest data point.Alternatively, if the integration is done continuously, such as by ananalog integrator with a decay time constant, the dV/dt sum couldlikewise be generated by feeding an electrical signal proportional tothe dV/dt into another analog integrator with a fixed decay timeconstant to generate a signal proportional to the running sum of dV/dtvalues.

This method is particularly applicable for machines using blades withrelatively coarse teeth and significant gullets between the teeth, suchas a 28-tooth circular blade with a diameter of 10 inches, wheretypically one tooth at a time would contact a person in an accident.This method has been found to distinguish contact between a person andsuch a blade from contact between wet or green wood and the bladebecause when the tooth of the spinning blade contacts a person andbegins to cut into the skin, the signal on the blade will drop quicklydue to the sudden connection of the human body's inherent capacitancevia the conductive contact between the tooth and the person. Similarly,the signal will rise sharply when the tooth breaks contact with theperson. The length of the short sum is typically adjusted to cover thetypical time interval that a tooth would be in contact with a finger sothat it sums both the dV/dts generated by the drop in signal when thetooth comes into contact with the finger and the dV/dts generated by therise in signal when the tooth leaves the finger. It has been observedwith a particular signal coupling to and from the blade that the signalon the blade typically drops in the range of 15-30%. When the toothmoves out of contact with the body, the signal will go up acorresponding amount. The method will detect and monitor how the signalamplitude changes, and add the changes together to arrive at the shortsum, as described. In contrast, when the tooth of a blade cuts into wetor green wood, for example, or when the tooth moves out of contact withwet or green wood, it has been observed that the change in the signal isnot as sudden even though the total amplitude change of the signal overtime may be similar to the amplitude changes seen when contacting aperson. Thus, the sum of dV/dt values resulting from contact with aperson is typically greater than the sum of such values resulting fromcontact with wet or green wood over the period observed by the shortsum.

As explained above, one implementation of the short sum method sums the16 most recent dV/dt values, resulting in a 96 microsecond window. Thatwindow corresponds roughly to the time it takes one tooth on a 10 inchblade spinning at 4000 rpm to move into and out of contact with a fingerin what is believed to be a typical accident scenario. It has beenobserved from experiments conducted with a hot dog acting as a fingerthat the sum of dV/dt values for hot dog contact is significantlygreater than the sum of dV/dt values for wet or green wood contact. Thethreshold at which the sum of the dV/dt values will trigger the reactionsystem will be set empirically to optimize the performance of the methodin any given machine. As a starting point, the threshold may be set at30% of the baseline integrated data value, or in other words, the normalvalue if nothing is touching the blade.

Using a window of roughly 96 to 100 microseconds to look at the shortsum allows the method to focus on changes in the signal rather than onthe amplitude of the signal itself. This is useful when cutting wet orgreen wood, especially wet plywood, wet particle board, or other gluedwood products. Those types of materials can create a dielectric effectby being adjacent the blade and the dielectric effect may cause theamplitude of the signal to change in proportion to the amount ofmaterial around the blade. Thus, looking simply at the amplitude of thesignal may in some cases result in the detection system determining thatthe blade has contacted a person when in reality it has contacted wet orgreen wood. The rate of change of the signal, however, is typically moregradual when cutting wet or green wood than it is when the bladecontacts a person, especially when accumulated over a period of timerather than looked at instantaneously, so the dV/dt values are differentand the cumulative sum of the absolute values of dV/dt values over anappropriately selected interval allows the detection system todifferentiate those contacts.

The method disclosed in FIG. 3 may also be adapted for blades withrelatively fine teeth, where typically more than one tooth at a timewould contact a person in an accident, such as with a 200-tooth plywoodblade. With such a blade, it has been observed that the overallamplitude of the signal will drop both when cutting into a person andwhen cutting into wet or green wood, especially wet, glued wood.However, the overall drop in amplitude will be accompanied by manysudden and frequent changes when cutting into a person because of thenumber and nature of teeth contacting the person's body. When cuttinginto wet or green wood the overall drop in amplitude is accomplished bya relatively smoother decline. Thus, the accumulated dV/dt values overtime when cutting into a person with a fine tooth blade will be greaterthan when cutting into wet or green wood. With this in mind, the methoddisclosed in FIG. 3 may be adapted so that the sum of the absolutevalues of dV/dt is taken over a longer interval, and this sum may bereferred to as the long sum. The window or number of data points summedby the long sum may be chosen to correspond to the approximate time forone tooth and one gullet on a coarse-toothed blade (e.g. a 28-toothblade) to move past a given point. Such a window would be approximately300-750 microseconds, or around 50-125 data points and more preferablyaround 70-90 data points sampled at 6 microsecond intervals. The windowis preferably chosen so that at least one complete period of toothstrikes would be included in the window, even on a coarse toothed blade.This effectively averages the rate of change of the signal over the timeperiod of a single tooth when cutting with a coarse tooth blade. Thislong sum is most responsive to the presence of many smaller fluctuationsin the signal, such as occur when many small teeth are contacting afinger versus a single large dip in the signal seen with coarser blades.

Typically the detection system will include both the short and long sumsoperating in parallel, and the system will trigger the reaction systemif either sum exceeds its predetermined threshold. In other words, thedetection system will sum the absolute values of dV/dt for the first 16data points and determine whether that short sum is greater than a firstthreshold. The method will also sum the absolute values of dV/dt forroughly the last 80 data points, and determine whether that long sum isgreater than a second threshold. If either threshold is met or exceeded,then the method will trigger the reaction system. If not, the methodrepeats for each new data point.

Thinking of the method in terms of processing in a DSP ormicrocontroller implementation, the processor will store running totalsfor the short and long sums. Each time a new data point is calculated;the method will add that data point to each of the running sums andsubtract the oldest data point. If either sum ever meets or exceeds itsrespective threshold, then the method will trigger the reaction system.It should be understood that the method does not need to trigger thereaction system in response to a single sum being over the threshold. Itmay also be desirable to require some number of sequential sums to beover a threshold value or for some proportion of recent sum values to beover a threshold value. In addition, it is possible to have thetriggering of the reaction system be based on how much the sums exceedthe threshold value. For instance, exceeding the threshold value by 50%may cause an immediate trigger, whereas the system may require threeconsecutive values over the threshold if the values exceed the thresholdby only 1%.

Detection subsystem 22 may also implement a third method of detectingcontact between a blade and a person. This third method is representedgenerally in FIG. 4 and may be referred to as a method for detecting arelatively slow drop in the electrical impedance of the blade. First, asignal is imparted to a designated portion of a machine, such as to theblade of a table saw, as shown at 130. The method then detects thesignal at 132 and converts the signal into an integer proportional tothe amplitude value at 134. The method then checks to see if theamplitude value is lower by a predetermined amount than any amplitudevalue during a designated prior interval of time, as shown at 136. If itis, then the method triggers the reaction system, as shown at 138. Asdiscussed previously, it is not necessary to look at the amplitude ofthe signal on the blade and any other electrical signal property relatedto the electrical impedance of the blade could equally be used.

In one embodiment, every 6 microseconds (or some other predeterminedtime interval) the method detects the signal on the blade and convertsthe signal into an amplitude value. Every 768 microseconds the methodwill collect 128 amplitude values (one value every 6 microseconds) andstore the lowest value in a bin. A predetermined number of bins will beused to store the low values over successive periods of time. For,example, the method may employ ten bins numbered 1 through 10, with bin1 storing the lowest amplitude value over the 728 microsecond intervalfrom time t₀ to t₁, bin 2 storing the lowest amplitude value from thenext time interval t₁ to t₂, bin 3 storing the lowest amplitude valuefrom time t₂ to t₃, and so forth with bin 10 storing the most recentlowest amplitude value from time t₉ to t₁₀. The method will then comparethe current amplitude value with the values in a prior interval, such asin bins 1 through 5. If the current amplitude value is lower than thelowest amplitude value in bins 1 through 5 by a predetermined amount,such as 15-30%, then the method will trigger the reaction system.

The method compares the current value with a prior, non-adjacentinterval of time in order to be able to detect when a person comes intocontact with the blade relatively slowly. If a person moves into contactwith the blade slowly, the signal will drop but the drop could berelatively gradual. In that case, the lowest amplitude value in relationto the most recent bin may not be sufficiently different to trigger thereaction system. By comparing the current amplitude signal with a prior,non-adjacent time interval, the method is better able to detect moregradual drops in the signal. Thus, the method may compare the currentamplitude signal with those in bins 1 through 5 rather than those inbins 6 through 10. Each new amplitude value is compared with the signalsfrom a prior interval, and the amplitude values for the prior intervalare updated as new amplitudes are calculated. For example, using theintervals and bins discussed above, every 728 microseconds the value inbin 1 is discarded, the values in bins 2 through 10 are shifted to bins1 through 9, and a new value is added to bin 10. If the currentamplitude value is lower than the lowest amplitude value against whichit is compared by a predetermined threshold or amount, then the methodwill trigger the reaction system, as explained. The threshold amount andthe desired time interval are empirically determined throughexperimentation and observation. It is believed that one appropriatethreshold is roughly 15% to 30% lower than the low values expectedduring typical operation of the machine. It is believed that onesuitable interval is roughly 5 milliseconds, ending approximately 5milliseconds before the time the current amplitude value is determinedfor a saw blade spinning at 4000 rpm.

Some detection subsystems may include a gain control to maintain thesignal on the blade at a desired amplitude. If the signal on the bladedrops for whatever reason, including if the signal drops because aperson contacted the blade, then the gain control tries to raise thesignal to maintain a target level of the signal on the blade. If thesignal on the blade is higher than the desired amplitude, then the gaincontrol tries to lower the signal. Watching the gain control forinstances when it tries to raise the signal on the blade can serve asthe basis for a fourth method of detecting contact.

FIG. 5 shows an embodiment of this fourth method of detecting contact.First, a signal is imparted to a designated portion of a machine, suchas to the blade of a table saw, as shown at 140. A gain control tries tomaintain that signal by periodically sampling the signal and applying adrive control scale factor to adjust the signal when necessary, asrepresented at 142. The method compares the most current drive controlscale factor with prior drive control scale factors to see if thecurrent factor has changed, as shown at 144. If the factor has increasedby more than a predetermined threshold, then the method triggers thereaction system, as shown at 146.

In one implementation, the gain control produces a drive control scalefactor four times during every 768 microsecond interval, and the methodcompares the most current drive control scale factor to one of the drivecontrol scale factors from each 728 microsecond interval during theimmediately prior 5 milliseconds. The system is preferably configured tohave different thresholds for different historical values of the drivecontrol scale factor. Specifically, older historical values of the drivecontrol scale factor would be larger than for the more recent values.Thus, the threshold for the difference between the current value and thehistorical value from 2 milliseconds ago may be 1.8 times as large asthe threshold for the change from 1 millisecond ago. The predeterminedthresholds at which the method will trigger the reaction system may beset empirically. This detection method is particularly applicable forsituations where the gain control may otherwise mask a drop in amplitudecaused by a finger briefly contacting the side of a blade.

It should be noted that the threshold values for some or all of theabove contact detection schemes may be scaled based on operationalparameters or conditions experienced by the system. For instance, it maybe desirable to scale down one or more of the long sum, short sum andfalloff thresholds as the AGC increases the drive level. If thethreshold values are scaled down inversely proportional to the drivelevel—i.e. if the drive level is doubled by the AGC, then the thresholdswould be reduced by a factor of two—then the relative sensitivity of thesystem to detection of contact will be maintained relatively constanteven when cutting wet wood, which would otherwise reduce the sensitivityof contact detection.

It may also be desirable to scale the thresholds at a level that is lessthan inversely proportional to the drive signal to cause the contactdetection sensitivity to adjust to only partially compensate for theinherent change in sensitivity that would otherwise occur when cuttingwet wood. Reducing or adjusting the sensitivity based on the apparentelectrical impedance of the blade may be desirable to increase theresistance of the system to false trips while cutting wet wood.

Alternatively, if no AGC is used, the automatic adjustment of thresholdvalues could be implemented by scaling down the thresholds proportionalto the average value of the signal detected on the blade. Under thisscenario, as wet wood decreased the apparent impedance of the blade,thereby causing the amplitude of the signal on the blade to drop with aconstant drive signal, the thresholds would scale down correspondinglyto maintain a relatively constant threshold sensitivity. In either case,it should be understood that scaling the sensitivity could beimplemented by directly adjusting the numerical value of the sensitivityor by making a corresponding adjustment to scale the signal level by acorresponding amount. By way of example, it is roughly equivalent toeither divide the threshold value by two or multiply each dV/dt value bytwo prior to adding them to the sum.

In addition to scaling the threshold values to maintain a relativelyconstant sensitivity, under some circumstances it may be desirable toscale the sensitivity of the system based on the noise currently beingexperienced by the system. For instance, if the noise level is high, butbelow the threshold to be detected as contact, the threshold forregistering contact can be raised to increase the resistance to falsetrips from such noise. Then, when the noise level is lower, thethresholds can be lowered to thereby improve the sensitivity to contact.By way of example, the threshold for detecting contact with the long sumtechnique may include a term that is proportional to the recent valuesof the long sum. More specifically, for instance, the long sum thresholdcould be set adaptively to be equal to one half of a nominalnon-adaptive threshold+one half of the average value of the long sumover the last 5 milliseconds. In this way, the sensitivity when noiselevels are low, as evidenced by a low average value for recent longsums, will be higher while maintaining good resistance to false tripswhen noise is higher, such as when cutting wet wood.

The above described types of adjustment of the contact detectionthreshold or sensitivity can be described as an active or adaptivesensitivity contact detection system since the sensitivity is adaptiveto the current conditions experienced by the system. Various portions ofthis adaptive sensitivity contact detection system are illustrated andembodied in the attached code.

The four methods disclosed above may be implemented together in onedetection subsystem. Implementing all four methods improves thelikelihood of detecting contact between a person and a designatedportion of a machine in a variety of accident scenarios. It alsoimproves the ability of a detection subsystem to distinguish contactbetween a person and the blade from contact between the blade and othermaterials such as wet or green wood. If any one of the four methodsdetects a contact, then the detection subsystem would trigger thereaction system to minimize any potential injury. It should also beunderstood that the detection subsystem could look for a particularpattern or combination of over-threshold values from one or more of thedifferent methods to further enhance the resistance to false triggers ofthe reaction subsystem.

The parallel operation of two or more detection systems, as describedabove, can be described as a multifaceted contact detection system. Itshould be noted that the so-called parallel or multifaceted operationdoes not require simultaneous execution on a microprocessor instructionlevel, although that could be possible with some processors, but ratherto the property that the detection schemes are operating to look at thedata associated with a single contact event in different ways, eventhough the data each scheme is looking at could be different. Forinstance, one scheme could be looking at the phase of the signal on theblade and the other could be looking at the voltage characteristics.Therefore, the computations associated with determining contact may becarried out sequentially for each scheme one after the other orpartially interspersed.

In the case where the thresholds in the different schemes areinterdependent in some way, such as where the fall off threshold isscaled based on the recent values of the long sum, the multiple paralleldetection schemes can be described as being selectively interrelated inthat the output of one scheme is dependent on the operation of the otherscheme or schemes.

It is important to again emphasize that although the methods describedabove have been discussed primarily in terms of amplitudes of a signalon a blade or of a drive control scale factor, the methods also can beimplemented by looking at other aspects of the signal or the gaincontrol. More generally, as stated, the methods are monitoring theelectrical impedance of the blade, and the amplitude of the signal andthe drive control scale factor are related to that impedance. Themethods could be implemented with values related or proportional toother characteristics of the impedance, such as the phase of the signalon the blade or the gain control, current delivered to the blade, or anyother variable related to the electrical impedance of the blade.

In the event of a contact event, it is possible to store some or all ofthe data associated with that contact event to a permanent memory, suchas flash. This flash could be either built into the DSP or an externalchip. Whatever type of permanent memory device is provided, it could beimplemented inside the cartridge or as some other part of the electricalsystem connected to the cartridge and could be permanently installed inthe cartridge or elsewhere on the machine or implemented as a removablecomponent. Generally speaking, it is desirable to move the datacurrently stored in RAM to the flash so that the data can besubsequently downloaded to provide a picture of the status of the systemat the time contact was registered and thereby information on whatcaused the contact event. By having users return or provide access tothe data stored in the fired cartridges or other element, it is possibleto download and analyze the stored data. A method is thereby provided ofacquiring data of actual human contact events, which is not otherwiseeasily feasible to acquire due to the injury that results from contact.In addition, if the operator reports that no contact occurred, beingable to look at the historical values stored in RAM provides someinformation on what might have caused the false trip so that correctiveaction can be taken. Thus, the described system can provide informationon the cause of false contacts or actual contact events. The method ofacquiring data may be described as follows: A method for acquiring datarelated to the triggering of a system designed to detect and triggerupon a dangerous condition between a human and a dangerous portion of awoodworking machine, the method including: distributing a plurality ofsuch systems; detecting a trigger of the system; storing data associatedwith the trigger in a permanent memory device associated with thesystem; and reading the data from the permanent memory device. Thismethod may further include the step of recovering at least a portion ofthe system, wherein the recovered portion includes the permanent memorydevice. The permanent memory device may be housed in a replaceablemodule, and the method may also include the step of providing a freereplacement module in exchange for a module that has trigger data storedtherein.

The methods discussed above may be implemented in power equipment innumerous ways through a combination of hardware, firmware and/orsoftware. In saws, it is often desirable to incorporate the hardware,firmware and/or software into a replaceable brake cartridge, such ascartridge 80 discussed above. By so doing, the hardware, firmware and/orsoftware may be revised or updated simply by changing the cartridge.

FIG. 6 shows an example of using a DB15 connector to connect theelectronics in the cartridge to a power supply, to a sensor assemblycomprising arbor electrodes (similar to contact detection plates 44 and46 discussed above), to switches, to a motor, and to an output displaysuch as LEDs. Transient voltage suppressors U18 and U19, as well asisolation resistors RN2 provide static discharge protection.

FIG. 7 shows an example of voltage regulation circuit that may be usedwith the cartridge. It shows the use of local regulation on the boardand isolation of the analog power rail from the digital power rail,preferably by a small series resistance R11 and shunt filteringcapacitors C5D and C3. DSP_ADC06 is connected to an analog to digitalconverter (ADC) input on a DSP to provide monitoring of the power supplyfor the cartridge.

FIG. 8 shows a digital signal processing (DSP) microcontroller used tocontrol the circuits described herein. The DSP includes a multi-channel,internal, analog-to-digital converter (ADC) input and internalpulse-width modulator (PWM) outputs, as well as clock, timing, RAM, ROMand flash memory functions. FIG. 9 shows a provision to program the DSPand flash memory from an external source, such as through the DB15connector shown in FIG. 6. The cycle time of the DSP is selected as 25nanoseconds, defined by crystal Y1 shown in FIG. 10. A reset chip U6,shown in FIG. 11, is used to hold the DSP in reset unless the powersupply is at proper operating voltage. FIG. 12 shows a phase lock loopused with the DSP.

A switch SW1, shown in FIG. 13, is one means of passing information tothe DSP. For example, a cartridge may be constructed so that switch SW1is not closed unless the cartridge is properly positioned and installed.In that case, switch SW1 would signal to the DSP whether the brakecartridge is properly installed and positioned in the saw. If thecartridge is not properly positioned, the DSP would not allow the saw torun.

FIG. 14 shows a circuit for a 500 kHz driver. The objective of thiscircuit is to generate a 500 kHz sine wave whose amplitude iscontrollable by firmware. This sine wave signal drives the capacitivecoupling electrode to impart a signal onto the blade. Preferably, thesine wave signal should be adjustable over a range of approximately3Vp-p to 30 Vp-p (3 to 30 volts peak-to-peak). It is also preferablethat it be possible to turn off the driver completely. A suitableadjustable voltage resolution is about ⅛% of full scale, although othervalues could be used. In addition, it is desirable that the outputvoltage be known to an accuracy of a few percent in order to detect thedistance between the blade and the brake pawl by measuring the amount ofsignal induced on the pawl for a known voltage level on the blade.Detecting the blade-to-pawl spacing is one way to insure the cartridgeis properly installed and to insure that a blade with a predetermineddiameter is installed in the saw.

The basic approach used in the driver is to generate a 500 kHz squarewave source of variable amplitude, and drive a resonator to create thesine wave output. Note that a 1Vp-p square wave has a sine wavefundamental component of about 1.3Vp-p. Thus, the first problem is howto generate a low impedance square wave of variable amplitude.

A simple approach would be to use a variable pulse width modulatedsignal at 500 kHz, and vary the duty cycle of the PWM to achieve thedesired average amplitude. However, in some applications, this approachmay not achieve the desired resolution in amplitude.

In the depicted embodiment, the power supply rail can vary between 5.5to 7 volts and the amplitude output should be fairly independent of themomentary changes in the rail voltage. It will be seen below thatbecause of the chosen Q of the resonator it will be necessary for thissquare wave to have a maximum amplitude of close to 5Vp-p. Thecombination of these two factors lead to the topology of using avariable current source driving a resistor to create a variableamplitude voltage, switching this voltage on-off by shunting the currentwith a switching transistor, and connecting the resulting variableamplitude square wave to emitter follower outputs that have a lowimpedance output. Referring to the schematic shown in FIG. 14, thevariable current source is formed by Q4/Q3 a/Q3 b, the resistor is R29,the shunt switching transistor is Q2, and the output emitter followertransistors are Q24 a/Q24 b.

The variability of the current source is provided by a pulse widthmodulated clock signal from the microprocessor driving an RC circuit,resulting in a variable voltage. To get sufficient resolution in thevoltage, two PWM outputs (DSP_IOPB2 and DSP_IOPA7) are used, scalingtheir values and summing them together with resistors R34 and R130.Capacitor C24 provides sufficient filtering of the ripple of the PWMsignal as well as the desired response time of the driver controlcircuit. Transistor Q4 and R35 convert this variable voltage into avariable current that is then mirrored and scaled by the Q3 a and Q3 bcircuitry. The result is a variable current source that will operateclose to the positive supply rail or to ground, providing the necessaryoutput voltage. The Vbe drop of Q4 introduces a dead zone at the low endof the transfer function between PWM code and output current, so R129provides an offset to minimize this dead zone. There will be some effectof temperature on the dead zone, but the actual driver output level willbe adjusted in a feedback control loop, so the dead zone only needs tobe a small percentage of the total range to have minimal affect on thegain of the circuit. There will be a second order effect of the powersupply rail variation changing the current due to the Early voltage ofthe transistors, and this is compensated out by R33.

The switching transistor Q2 is chosen to have a low saturation voltage,and is driven with a base RC network that will turn it off quickly butwill avoid excessive charge storage in the base-emitter junction. AMOSFET having a slightly lower voltage when turned on may be used, butmay also have a higher cost. It should be noted that the driver can beturned off by simply stopping the switching of Q2.

Complementary emitter follower transistors provide a low impedanceoutput that drives near the rails. The RC network of C19 and R30 permitthe base of Q24 b to be driven below ground to get the widest possibleoutput voltage.

The resonator is formed by L1 and C21, C20, C23, and any reactanceloading at the DRIVE_OUT node. Economics motivate the use of a 5%tolerance on the inductor and capacitors, with additional uncertaintydue to temperature. In addition, the capacitance looking out theDRIVE_OUT port can vary between a nominal level when the blade is notloaded to a higher level when sawing wet wood, and it is desired to havethis variability affect the amplitude of the signal at DRIVE_OUT by onlya few percent. These factors, combined with the desire to generate thevoltage range of 3 to 30Vp-p at DRIVE_OUT, led to the use of a Q=10 inthe resonator. This Q is set by the resistor R27 combined with thetypical loss in the inductor. If the resonator is centered at 500 kHzthen the maximum output voltage will be in excess of 50Vp-p. However,under worst case conditions of resonator tuning, the output voltage canstill achieve 30Vp-p. Multiple capacitors were needed to define theexact center frequency of the resonator from standard values. C20 alsois used in a detector circuit with D11, and can be 10% tolerance becauseit has a small reactance in comparison with C23.

Diode D11 creates a level detector that permits measurement of theoutput level of the driver to within about 1 percent if 0.1% toleranceresistors are used in the R31/R32 divider. The DC component of thevoltage at the cathode of D11 is directly proportional to the sine wavelevel at the driver output; a low-pass filter formed by R31, R32, C22and C2B roll off the 500 kHz to provide just the DC component to the A/Dconverter. Other techniques are possible for measuring the output levelof the driver, including direct sampling of one of the node waveforms byan A/D converter.

FIG. 15 shows an integrator circuit that may be used in theabove-described methods. The objective of the integrator is to provide away to periodically measure the amplitude of the driven signal. In thedepicted embodiment this driven signal is at 500 kHz and the measurementperiod is every 6 microseconds. This circuit is preferably designed tominimize immunity to spurious signals at other frequencies, as well asto provide relative immunity to electrostatic discharge from events suchas the charging of the blade/arbor from a rubber drive belt or cuttingof non-conductive materials. However, it should be understood that thisfunction could be accomplished many different ways including with a peakamplitude detector, a power detector, or direct sampling of the signalwith an A/D converter to measure the amplitude of the signal. Also,although the measurement in the present circuit is carried out atdiscrete time intervals, it should be understood that such a measurementcould be carried out continuously.

The topology of the circuit is to amplify and full wave rectify thesignal at the node marked DRIVE_SENSE, and then drive an integrator thatis sampled at the end of the measurement period and then reset. Themeasurement period was chosen to be synchronous with the drive signaland to be an integer multiple of the number of cycles of the drivesignal to minimize ripple in the measurement. The integration isperformed by current source Q8 driving integrating capacitor C30. U10has sufficient current carrying capability to quickly reset the voltageon C30 to zero after each measurement. Q10 is a phase splitter to splitthe measurement signal into two components 180 degrees out of phase.These outputs are followed by the Q7 and Q11 stages, whichnon-symmetrically amplify only the positive peaks coming through theirinput coupling capacitors. The net effect is a full wave rectifiedversion of the measured signal appearing at R46, which drives theintegrator. It should be understood that there are a number of othercircuits that could provide this full wave rectifier function, includingdiode rectifiers. Q9 is a buffer that presents a high impedance load atits input and a low impedance source at its output to drive a filter.Q25 a and Q25 b in conjunction with C60 and C26 form a filtered lowimpedance power supply.

Amplitude and frequency filtering is used throughout this circuit tominimize interference by spurious signals. A high pass filter is formeddue to the capacitive coupling of the arbor electrode and the inputimpedance of this circuit. Diodes D14 and D23 provide two levels ofclipping of the input voltage to prevent ESD spikes from damaging othercircuit elements and disrupting the measured amplitude. R47 and C74 forma low pass filter to block high frequency noise. Coupling capacitorssuch as C32, C33 and C27 form high pass filter elements that reject lowfrequency spurious signals. Network C73, L2 and C72 is a band passfilter. The combined effect of these coupling elements, low pass, andband pass filters is an overall band pass filter function centered onthe expected frequency of the sensed signal, typically about 500 kHz.This reduces the opportunity for noise to get into the circuit anddisturb the resulting measurement.

In addition to the filtering provided by analog elements in the circuit,the attached code implements a type of digital filtering on the signalto further reduce noise. In particular, the integrator output isdigitized just prior to reset and the result is a 10 bit binary number,typically around 600 if no perturbations affecting the impedance of theblade. Changes in this value are clipped so that they are recognizedonly to the extent they are smaller than a threshold value—MaxStep—inthe code or to the extent that the next value changes in the samedirection as the first change. Typical values of MaxStep are between 1and 50 and will depend on the specific impedance and noisecharacteristics of the saw.

In addition to the above described digital filtering, the attached codealso implements a type of hysteresis filtering whereby changes inintegrator values are only recognized to the extent they exceed athreshold step. The threshold step can be made dependent on whether thestep is a positive or negative change and whether the step is in thesame direction or opposite direction as the prior step. Since the dV/dtvalues are the changes in integrator count from sample to sample,eliminating small variations in the integrator count reduces the effectof noise on the sums that are used to detect contact, by eliminating theeffect of many small changes and instead only recognizing relativelylarge changes such as might be induced by a contact event.

In some implementations, such as in a table saw, a metal brake pawl ispositioned adjacent the perimeter of a blade and it is desirable todetect whether the pawl is sufficiently close to the blade or otherwisepositioned properly. It is also sometimes desirable to confirm that ablade with a predetermined diameter is installed in the saw. Thesefunctions can be accomplished by detecting blade-to-pawl spacing. Thiscan be accomplished by a variety of techniques including but not limitedto mechanical, optical, electrical and/or magnetic means.

FIG. 16 shows one circuit that may be used to electrically detectblade-to-pawl spacing. The depicted circuit makes an accuratemeasurement of the portion of the drive signal going to the saw bladecoupled to the metal brake pawl. The DSP uses the ratio of the drivesignal to the detected signal on the pawl to determine a transferfunction, and infers the gap with prior knowledge of the relationship ofgap distance to the transfer function. The Pawl Sense circuit is anamplifier whose input is connected to the metal brake pawl and whoseoutput drives the A/D converter. The A/D converter directly digitizesthe pawl signal and determines its amplitude in firmware, although othertechniques are possible such as using a detector circuit.

This amplifier should have a very high input impedance as it providesthe load for the approximately 2 pF capacitive coupling between the sawblade and the pawl. It also should have an output impedance that is lowenough to drive the A/D converter input without error. It is desirableto have some gain that is known and stable over environmental conditionsto within a few percent. Finally, it should also withstand staticdischarge of the saw blade without damage.

Static protection is afforded by R62 and low capacitance diodes D17.Diodes D29 at the output help protect the A/D input from discharges thatwould otherwise overdrive the voltage into the A/D.

High input impedance requires both a high input resistance and low inputcapacitance. This is achieved first by using a high gain transistor forQ26 a, which means that the associated bias resistors can be high andthe low frequency impedance looking into the base is high. In addition,a bootstrap technique is used to cancel the collector-base capacitanceof Q26 a which would otherwise lower the input impedance: an in-phaselow impedance signal at the emitter of Q26 b is coupled through C36 tothe collector of Q26 a to make that collector be almost perfectlyin-phase with the signal at the base, canceling the capacitance betweenthe two.

Q26 b provides gain for the pawl signal. Q12 is a unity gain buffer thatis operated at such a current so as to provide a low output impedance todrive the A/D converter input.

FIG. 17 shows a circuit used to power motor control relays. One featureof the relay driver is the use of series redundant pass transistors (Q30and Q31) which are preferably controlled independently, and fromseparate microcontroller ports, to keep a single failure from turning onthe relay that powers the saw motor. In addition, the microcontrollerreset controller locks one of the pass transistors off through D26during power-up or power-down to further ensure that the relay does notturn on at the wrong time. The microcontroller can test the integrity ofthe two pass transistors via the feedback path between the relay coiland DSP_IOPC7: one pass transistor can be turned briefly on at a timeand the microcontroller can tell if just one pass transistor is enoughto provide power to the relay, indicating that the other has failedshorted.

FIG. 18 shows a circuit used in firing subsystem 76, discussed above. Inthis circuit, an SCR is used to discharge a high voltage capacitor C41through a fuse wire in order to melt it open and thus release a storedenergy source such as a spring to drive the brake pawl against the sawblade. In this circuit, the fuse wire extends between locations TP23TP24and TP25TP26. In order to melt the fuse wire, the SCR must conducthundreds to thousands of amperes for a few tens of microseconds. Toensure that the SCR turns on fully and quickly it is desired to deliverin excess of 1 ampere of current quickly into the gate. This is donewith transistor Q18 that is configured as a current source. It may benecessary to limit the amount of time this high gate current flows inorder to avoid over-dissipating the SCR gate or the drive circuits. Thisis achieved by supplying the majority of the SCR trigger current withC39, so that the current drops to a much lower value after the initialcurrent surge. This time limit can be additionally achieved by limitingthe amount of time that the control line from the microcontroller isasserted. The control line labeled FIRE_INHIBIT is connected to themicrocontroller reset circuit and serves to disable the SCR fromtriggering during power-up or power-down of the board.

In order to confirm the functionality of the circuit, means are providedto test trigger the SCR at a low voltage on C41, typically 3.3V, so thatthere is insufficient current to overstress the fuse wire but enoughcurrent to permit verification that the firing circuit is functional.High voltage capacitor C41 is charged up to typically 3.3V by the actionof the Q32 a/Q32 b/D24 circuit. Then the SCR is triggered and thevoltage on C41 is monitored, either through a voltage divider (leadingto DSP_ADC07 or DSP_ADC02) or through C75 which is an AC coupled circuitthat permits monitoring the change in voltage on C41 with time.

Means is also provided to measure the capacitance of C41 to insure it isfunctioning as intended. This can be accomplished without interferingwith the ability of the capacitor to deliver sufficient current to meltthe fuse wire, so it can be performed repeatedly throughout the life ofthe capacitor even if the circuit is being called upon to providecontinuous protection. With the capacitor charged to a target voltage,typically around 180V, a momentary load is applied to the capacitor andthe resulting change in voltage is monitored and the capacitance can becalculated from this voltage change. If this load is applied for a shortperiod of time, typically 5 milliseconds or less, then there will onlybe a small percentage change of voltage on the capacitor C41, typically½ percent, and so sufficient charge remains available in C41 to melt thefuse wire if called upon to do so. The load is provided by resistorsR171/R106/R70 and MOSFET switch Q21, with a provision for measuring thecurrent through those resistors by measuring the voltage across resistorR91. The voltage change across C41 resulting from that load ispreferably monitored through AC coupling capacitor C75 because only afew volts change on C41 gives a full scale change to the A/D converterinput through this path, and so is more sensitive to the voltage changeson C41 than through a voltage divider path such as goes to DSP_ADC02.Various combinations of applying a load and measuring the resultingvoltage change can be used to measure the capacitance of C41.Alternatively it is possible to calculate the capacitance of C41 bymonitoring the time it takes to charge up, or by monitoring the voltagecharacteristics with time during the low voltage discharge test.

Means is also provided to prevent discharge of C41 out through the boardconnector, such as when the replaceable cartridge is removed from thesaw, which could result in a shock to a person handling the cartridge.This is accomplished through redundant series diodes D22.

The exemplary circuits shown in FIGS. 6 through 18 may all beimplemented on a circuit board housed in a replaceable cartridge, asexplained above.

FIG. 19 shows a power supply circuit that may be implemented in powerequipment such as a table saw and is suitable to use with the abovedescribed cartridge circuit. The power supply would receive line power,transform it as necessary, and supply power to the brake cartridge. Inthe circuit shown in FIG. 19, a universal 110V/240V, 50 Hz/60 Hz inputswitching power supply is used to provide isolated power to thecartridge. There are two outputs of this supply: a typically 5.5V supplyto power most circuits on the cartridge, and a typically 200Vhigh-voltage supply used to charge capacitor C41 (shown in FIG. 18) thatprovides the energy to melt the fuse wire, as discussed above. Feedbackis provided to the switching controller integrated circuit U1 via anopto-isolator circuit U2 that monitors when either supply is above itsset point.

The high-voltage supply can be switched on or off by Q3. In addition theoutput current is limited by Q4 and Q5 to typically 30 mA. This ensuresthat C41 on the cartridge board reaches its operating voltage morequickly than if a simple current limiting resistor was used.

It is desirable to prevent the brake cartridge from firing if a persontouches the blade when it is not spinning. This is accomplished bymonitoring when the blade is spinning, and disarming the brake cartridgeif the blade is not spinning. In the depicted circuit, the output from azero-cross detecting circuit (U3) is used by the DSP or microcontrolleron the cartridge board to detect when the saw motor (and thus the sawblade) is turning slower than a certain rotational rate. Thisinformation is used to disarm the brake pawl from firing when the bladeis essentially not spinning. This zero-cross detector measures thereverse-EMF of the motor and is coupled to the microcontroller throughan optical coupler U4 for electrical isolation.

Relay K1 is used to switch power to the saw motor. To minimize arcingacross the relay contacts, a bypass circuit composed of Q1/Q2/U5 is usedto optionally bypass current around the relay contacts just before therelay contacts are closed and for a short period after the relaycontacts are opened. Because current flows in this bypass circuit foronly a fraction of a second before relay contact closure or aftercontact opening, less expensive low power components can be used thanwould be otherwise necessary if it carried the current for indefiniteperiods. It would be possible to control this bypass circuit from aseparate control line, but use of control lines can be minimized bymultiplexing this bypass control on the same line that energizes therelay coil. A low duty cycle pulse train, typically 10%, initially sentto the relay coil does not have sufficiently high voltage to energizethe relay, but gets detected through CR20 by SCR driver U5 which thenturns on SCRs Q1 and Q2 to bypass the relay contacts. After a shortperiod of time, typically 400 milliseconds, the duty cycle of the pulsetrain is raised to 100% to energize the relay coil. When it is time todeactivate the relay the duty cycle is dropped to a low value again,typically 10%, and held there for typically 400 ms while the relaycontacts open. Arcing across the relay contacts is thus minimizedbecause current is diverted around the contacts before they close orafter they open. In the case of larger motors, the relay may be usedjust to energize the coil on a contactor to control the larger motor,rather than controlling the motor directly.

Means is provided in the circuit depicted in FIG. 19 to dissipate staticcharge that may be present on the cartridge board when it is removedfrom its packaging and before it mates with its connector. The shield ofthe D-sub connector in the saw is connected to ground via resistor R5,thereby providing a discharge path. Static charge may then dissipatewhen the cartridge contacts the shield before it mates with theconnector.

A user interface between the operator and the microcontroller on thecartridge board is also provided in the power supply shown in FIG. 19.Switch contacts SW2 and J6 are available to notify the microcontrollerthat the operator wishes, for example, to turn on the saw motor oroperate the saw without the protective system enabled. Additionally themicrocontroller can notify the operator of status information by pullinglines J3-4 or J3-8 to logic low to activate indicators such as LEDs orbuzzers.

The circuit shown in FIG. 19 may be housed in a switch box mounted onthe saw. A possible switch box is described in U.S. Provisional PatentApplication Serial No. 60/533,598, entitled “Switch Box for Power Toolswith Safety Systems,” naming Stephen F. Gass, James David Fulmer andDavid A. Fanning as inventors, which application is hereby incorporatedinto this document by reference.

FIG. 20 shows a high level view of how the circuits discussed above maybe incorporated into a machine such as a saw having a relatively smallmotor (e.g., a 1.5 horsepower single phase motor). A power board 200,having a power supply circuit like the one disclosed in FIG. 19, may behoused in a switch box mounted on the machine. A ground connection andpower Lines A and B are supplied to the power board at connections J2-1,J2-2, and J2-3, respectively. A motor 202 is connected to power Line Band to connector J2-5 which connects to power Line A on the power board.The power board turns on the motor by supplying power to the motorthrough connector J2-5.

Motor 202 and line J2-5 are also connected to line J2-4 as shown. Thatconnection is used to sense whether a cutting tool in the machine isrotating, and the connection may be referred to as a zero-cross sensingconnection. This circuit is detailed above and additional detail isdisclosed in U.S. Provisional Patent Application Ser. No. 60/496,568,entitled “Motion Detecting System for Use in a Safety System for PowerEquipment,” the disclosure of which is herein incorporated by reference.

A cable 204 connects the power board to a connector 206. In the circuitsdiscussed above, cable 204 is preferably a nine-conductor cable with aground shield and connector 206 is a high-density, D-Sub style connectorwith 15 conductors.

Wires 208, 210 and 212 also connect to connector 206. Wire 208 is aground wire attached to a portion of the machine such as an arbor blockin a saw to insure a secure ground connection between the contactdetection circuitry and the structure of the saw. Wires 210 and 212connect to the electrodes used in the detection subsystem to impart asignal to a designated portion of the machine such as to the blade in atable saw, as discussed above.

One portion of connector 206 is mounted in the machine and anotherportion is mounted on a cartridge board 214 housed in a replaceablecartridge. The cartridge board may include the exemplary circuits shownin FIGS. 6 through 18, as stated. The connection between the twoportions of connector 206 is made when the cartridge is installed in themachine.

In the circuits discussed above, the female portion of the connectorwould preferably be mounted in the machine and the male portion wouldpreferably be mounted on the cartridge because the pins in the maleportion are more easily damaged and it is easier to replace thecartridge than it is to replace the female portion of the connectorwhich is mounted semi-permanently in the saw.

FIG. 21 shows how the circuits discussed above may be incorporated intoa machine such as a saw having a larger motor (e.g., a 3 to 5 horsepowermotor powered by either single or three-phase power). The connectionsbetween the power board, connector, cartridge board, electrodes andground are similar to those shown in FIG. 20.

The power board is connected at line J2-1 to ground and to power lines Aand B at J2-2 and J2-3, respectively. Power lines A and B and C are allconnected to a contactor 216 having a coil 218, and the contactor isconnected to motor 220. Coil 218 is connected to power line B and toconnector J2-5, which selectively connects to power line A through thepower board. Supplying power to coil 218 through connector J2-5 causesthe contactor to close and supply power to the motor.

Line J2-4 is also connected to motor 220 and is used to sense whether acutting tool in the machine is rotating. Again, this connection may bereferred to as a zero-cross sensing connection. A 100 k resistor,labeled by the number 222, also connects motor 220 to connector J2-3across the contactor, as shown, to provide a closed circuit path for thezero-cross sensing.

Two copies of a computer program that is one implementation of themethods discussed above are being submitted concurrently on two compactdiscs as a Computer Program Listing Appendix. That program isincorporated into this document by reference. The computer programcomprises a full description of the methods described herein, and isintended for operation with the circuits shown in FIGS. 6 through 19 and22. The program is most specifically written in assembly language to runon a Texas Instruments TMS320LF2407A digital signal processor or onother similar processors. The schematic for this implementation is shownin FIG. 22, which would replace the schematics of FIGS. 8-13. TheTMS320LF2407A has the capability to implement 64 k of external RAM, asshown, which provides the opportunity for enhanced debuggingcapabilities, but does so at the expense of occupying a larger area onthe circuit board. Therefore, it is believed that the TMS320LF2403A is apreferred device to use in a production cartridge where space is verylimited and the debugging capabilities are less important. Furthermore,if quantities justified the NRE cost, it is possible also to use aTMS320LC2402A, which is a ROM-coded device (no flash) that must beprogrammed at the time of manufacture, but is substantially cheaper oncethe initial ROM programming is completed.

It can be seen from examination of the attached computer code incombination with the above-described circuit that the disclosed systemimplements a wide variety of self-tests. In particular, the system teststhe SCR and the circuit that drives the gate of the SCR at power up ifthe voltage on the capacitor is low enough that the fuse wire will notbe degraded by firing the SCR and releasing the charge in the capacitor.The voltage as a function of time during the discharge is monitored tomake sure that the SCR, capacitor and discharge path have a sufficientlylow series resistance to cause the fuse wire to break if the dischargeoccurs at high voltage. Also, the time of the peak discharge currentrelative to the turn on pulse to the SCR is measured to monitored toverify the functionality of the SCR and gate drive circuitry.

After the SCR test is completed the high voltage energy storagecapacitor is charged to its operational voltage. The first test is thatthis capacitor reaches the correct voltage level to have sufficientenergy to burn the fuse wire. Once the capacitor reaches operationalvoltage, a short discharge under a known resistive load is carried out.The current in the discharge and the voltage drop on the capacitorduring the discharge are both measured and cross-correlated with theoperation voltage to insure that the capacitor has sufficientcapacitance to burn the fuse wire when charged to the operationalvoltage. In addition, the measurement of discharge current provides aredundant direct measure of the voltage on the capacitor. Thus, if theresistive divider network that is used to sample the voltage on the highvoltage capacitor is degraded in some way, the current measured duringdischarge will not match its expected value and an error can be set towarn the user of a failure in the circuit. This discharge test ispreferably carried out periodically whenever the saw is powered up toinsure that the high voltage capacitor is fully functional at all times.

As described above, the circuitry associated with turning on the relayto start the motor is redundant to provide the maximum reliability. Inaddition, the circuitry in association with the attached code provides atest to verify the operational condition of both transistors used tocontrol the relay as part of the relay turn on process each time themotor is started. Should either transistor fail the test, then systemwill set an error and the motor will not start. Use of redundant seriescontrol elements, and functional testing of those elements, to controlthe operation of the motor minimizes the chances that a single failurecan cause the motor to turn on in an uncontrolled fashion.

The bypass and start switches, described in the application incorporatedby reference above titled “Switch Box for Power Tools with SafetySystem,” that are part of the user interface also include a self-testfunction. In particular, when power is applied to the circuit, both thebypass and start switches must be in the off condition before an oncondition will register with the system. For instance, if the startswitch is inadvertently placed in the on condition prior to theapplication of power to the system, then when power is applied, an errorwill be set that will prevent the system from operating the motor untilthe start switch is cycled to the off position. The same is true of thestatus of the bypass switch. Also, the status of the switches is sampledmultiple times prior to accepting a change of state. For instance, inorder for the start switch to register as on, the system must sample thestart switch multiple times over the course of many milliseconds or tensof milliseconds and find the switch in the on condition each time torecognize the start switch as being in the on condition. As illustratedin the attached code, similar functionality is incorporated intorecognizing a turn off command on the start switch.

In addition, engagement of the bypass mode, wherein the contactdetection system is temporarily disabled, is controlled by the bypassswitch. In particular, as illustrated in the attached code, in order toengage the bypass mode, the bypass switch is turned and must be held fora first period of time prior to engaging the start switch. Then, oncethe start switch is engaged, the motor turns on and the bypass switchmust continue to be held on for a second period of time. If this patternis started but interrupted prior to completion, the motor will turn offif it is already on and the sequence must be started again from thebeginning. The use of an activation pattern for the bypass insures thatthe bypass system will not be engaged by accident and that there will beno mistake as to whether the bypass was correctly engaged. In addition,once the bypass is engaged, the LED's in the user interface areilluminated in a predetermined fashion to indicate to the user visuallythat the bypass is engaged. It should be noted that the bypass switch iskey controlled to allow a shop supervisor or other person to remove thekey to prevent the system from being used in the bypass mode.

Numerous checks are built into the contact detection signal path asdescribed in the circuit above when taken in combination with theattached code. First, the contact detection signal must generate adesired output at the integrator or the system will set an error and themotor will not start. Thus, if a wire is broken, or the blade isaccidentally grounded, the operator will not be able to start the motorand an error code will be flashed on the LED's in the user interface.

In addition, the various detection schemes are monitored and if abackground noise level causes the detection schemes to reach a levelclose to but not exceeding the trigger level, the system will shut offthe saw without triggering the braking system to warn the user that afalse trip is imminent. The system is configured to look at the levelregistered on the detection schemes as an average over a long enoughtime scale that actual contact events won't have any significant effecton the average. Therefore, it may be desirable to look at the averagedetection scheme level over a time of about 1 to 20 milliseconds ormore, and 5 milliseconds is believed to be a particularly suitableaverage window length. This averaging can be accomplished easily in amicrocontroller by use of a so-called infinite impulse response or IIRfilter, as shown in the code. The word average is used herein in a moregeneral sense than just the mathematically defined average and is thusmeant more to convey a measure of the characteristic level of a signalover a period of time, such as by the just-described IIR. It can be seethat tracking the response level of one or more of the detection schemesduring operation and interrupting operation if the tracked responselevel exceeds some threshold less than the contact detection threshold,provides a method of reducing the chance of a false trip.

In addition to tracking the response level of the detection schemes, theattached code is configured to track the drive signal level and shut offthe motor if the drive level required to maintain the sensed signal atthe target level exceeds some threshold. This can occur if a user triesto cut extremely wet wood, for instance. Monitoring the drive level andshutting the motor off if the drive level exceeds a threshold levelprovides yet another method of reducing the chance of false trip andprovides a method to insure that the motor only continues to operate solong as contact can reliably be detected. Given the various componenttolerances, it may be desirable to calibrate the nominal drive level atthe time of manufacture to provide a more accurate measure of the actualdrive level required under nominal conditions to have the sensed signalin regulation. The ratio of the current drive level to this nominaldrive level is the preferred control off of which the drive level motorturn off threshold and the contact detection threshold are adjusted.

The attached code also embodies tracking of the AGC regulation. Inparticular, the AGC is designed to maintain the nominal sensed signal ata predetermined level. If the sensed signal deviates from that level, anAGC error is generated. This error is tracked and averaged on a timescale longer than tooth strike events to insure that the AGC is inregulation. If the average or characteristic level of the AGC error isgreater than some threshold when computed over a sufficiently long time,then an error is set which prevents operation of the motor or causes itto turn off if it is already on.

As described above, the disclosed embodiment includes a circuit todetect the spacing between the pawl and the saw blade. This test is usedto insure that the operator doesn't attempt to operate the saw with ablade that is too small for the installed cartridge, such as an 8″ bladeinstead of a 10″ blade, or with a blade that has a non-conductive hub.This can also be used to insure that the brake pawl is positioned closeenough to the blade if a spacing adjustment is provided.

The voltage level of the power supply line powering the cartridge boardcircuitry is also monitored to insure that it is neither too high nortoo low. If the level goes too low, that may be indicative of a loss ordegradation in power in which case the motor can be shut off or blockedfrom starting to minimize the danger to the user under thosecircumstances.

INDUSTRIAL APPLICABILITY

The systems and components disclosed herein are applicable to powerequipment and to safety systems that detect human contact with powerequipment.

It is believed that the disclosure set forth above encompasses multipledistinct inventions with independent utility. While each of theseinventions has been disclosed in its preferred form, the specificembodiments thereof as disclosed and illustrated herein are not to beconsidered in a limiting sense as numerous variations are possible. Thesubject matter of the inventions includes all novel and non-obviouscombinations and sub-combinations of the various elements, features,functions and/or properties disclosed herein. No single feature,function, element or property of the disclosed embodiments is essentialto all of the disclosed inventions. Similarly, where the claims recite“a” or “a first” element of the equivalent thereof, such claims shouldbe understood to include incorporation of one or more such elements,neither requiring nor excluding two or more such elements.

It is believed that the following claims particularly point out certaincombinations and sub-combinations that are directed to one of thedisclosed inventions and are novel and non-obvious. Inventions embodiedin other combinations and sub-combinations of features, functions,elements and/or properties may be claimed through amendment of thepresent claims or presentation of new claims in this or a relatedapplication. Such amended or new claims, whether they are directed to adifferent invention or directed to the same invention, whetherdifferent, broader, narrower or equal in scope to the original claims,are also regarded as included within the subject matter of theinventions of the present disclosure.

1. A method for detecting contact between a user and a cutting tool on apiece of woodworking equipment, the method comprising: applying anelectrical signal to the cutting tool, wherein the cutting tool has anapparent electrical impedance dependant at least in part on whether thecutting tool is in contact with the user, and wherein the electricalsignal has an associated property related to the apparent electricalimpedance of the cutting tool; integrating the property to obtainintegration results that are dependant at least in part on the apparentelectrical impedance of the cutting tool over time; generating a valuebased at least in part on the integration results; and deciding therehas been contact between the user and the cutting tool if the valuecompares to a threshold in a predetermined way.
 2. The method of claim1, wherein the integrating is done with an analog circuit.
 3. The methodof claim 1, wherein the integrating comprises multiple integrations overdiscrete time intervals.
 4. The method of claim 1, wherein the propertyis digitized prior to integrating.
 5. The method of claim 1, wherein theproperty is related to the amplitude of the signal applied to thecutting tool.
 6. The method of claim 1, wherein the property is relatedto the phase of the signal applied to the cutting tool.
 7. The method ofclaim 1, wherein the property is related to the current associated withthe signal applied to the cutting tool.
 8. A method for detectingcontact between a user and a cutting tool on a piece of woodworkingequipment, the method comprising: inducing an electrical signal on thecutting tool by generating an electrical current between a drive circuitand the cutting tool, wherein the electrical signal induced on thecutting tool has a phase and an amplitude, wherein the cutting tool hasan apparent electrical impedance dependant at least in part on whetherthe cutting tool is in contact with the user, and wherein the electricalcurrent, phase and amplitude are dependant at least in part on theapparent electrical impedance of the cutting tool; sampling one or moreof the current, phase or amplitude a plurality of times at discreteintervals to generate multiple data points, wherein each of the datapoints is the result of a discrete integration; calculating a valuebased at least in part on more than one of the data points; and decidingthere has been contact between the user and the cutting tool if thevalue compares to a threshold in a predetermined way.
 9. A method fordetecting contact between a user and a cutting tool on a piece ofwoodworking equipment, the method comprising: inducing an electricalsignal on the cutting tool by generating an electrical current between adrive circuit and the cutting tool, wherein the electrical signalinduced on the cutting tool has a phase and an amplitude, wherein thecutting tool has an apparent electrical impedance dependant at least inpart on whether the cutting tool is in contact with the user, andwherein the electrical current, phase and amplitude are dependant atleast in part on the apparent electrical impedance of the cutting tool;sampling one or more of the current, phase or amplitude a plurality oftimes at discrete intervals to generate multiple data points;calculating a value based at least in part on more than one of the datapoints, wherein the value is based at least in part on a change in atleast one of the data points relative to a sum of a plurality of datapoints; and deciding there has been contact between the user and thecutting tool if the value compares to a threshold in a predeterminedway.